Durability-enhanced thermochromic film and method of manufacturing same

ABSTRACT

A thermochromic film includes a substantially transparent substrate, and a polymer matrix layer coated on the substrate, the polymer matrix layer having cross-linked molecule chains generated before the coating, and the polymer matrix layer being embedded with randomly dispersed thermochromic nanoparticles before the coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 17/028,397, filed on Sep. 22, 2020, titled “DURABILITY-ENHANCED THERMOCHROMIC FILM AND METHOD OF MANUFACTURING SAME”, now U.S. Pat. No. 11,713,385, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates generally to window insulations, and, more particularly, to a durability-enhanced thermochromic film for window retrofits.

With solar transmittance higher than 90%, as much as roughly 800 W/m² solar irradiation reaches the indoor environment through windows during daytime (K. Ismail, J. Henriquez, Thermally effective windows with moving phase change material curtains, Applied Thermal Engineering, 21 (2001) 1909-1923; and R. C. Temps, K. Coulson, Solar radiation incident upon slopes of different orientations, Solar Energy, 19 (1977) 179-184). The transmitted solar energy greatly reduces the energy consumption for heating in cold climates. However, excessive solar heating would result in increased cooling loads in hot climates, especially in summer. Smart windows that can dynamically adjust the transmittance of solar irradiation have been proposed as one of the most promising techniques to reduce the energy consumption of buildings (Y. Wang, E. L. Runnerstrom, D. J. Milliron, Switchable materials for smart windows, Annual Review of Chemical and Biomolecular Engineering, 7 (2016) 283-304; and H. N. Kim, S. Yang, Responsive Smart Windows from Nanoparticle-Polymer Composites, Advanced Functional Materials, (2019) 1902597). In a copending application, U.S. Ser. No. 17/028,301, filed by the common applicant and the disclosure of which incorporated by reference in its entirety, an optically-switchable thermally-insulating vanadium dioxide (VO₂) aerogel hybrid film (VAH) is disclosed for retrofitting windows. However, pristine VO₂ is thermodynamically unstable and can be easily oxidized to V₂O₅ when exposed to air for several months, which in turn dramatically reduces the solar modulation ability. A humid environment would also greatly accelerate the oxidation process.

Even though environmentally stable oxides like SiO₂, TiO₂, and ZnO have been used to increase the durability of VO₂ nanoparticles, in practical applications, when VO₂ nanoparticles transit periodically from monoclinic (M, a_M=5.75,b_M=4.52 Å,c_M=5.38 Å,β=122.6°) structure to tetragonal rutile (R, a_R=b_R=4.55 Å,c_R=2.86 Å) structure (Y. Cui, Y. Ke, C. Liu, Z. Chen, N. Wang, L. Zhang, Y. Zhou, S. Wang, Y. Gao, Y. Long, Thermochromic VO₂ for energy-efficient smart windows, Joule, (2018)), the interface stress between the VO₂ cores and oxide protection shells induced by the lattice structure transformation of VO₂ nanoparticles may result in the formation of micro cracks at the interface. The accelerated aging tests at a temperature of 60° C. and 90% relative humidity showed that the lifetime of the SiO₂ coated VO₂ nanoparticles were only about 72 hours due to the appearance of such cracks (Y. Chen, X. Zeng, J. Zhu, R. Li, H. Yao, X. Cao, S. Ji, P. Jin, High performance and enhanced durability of thermochromic films using VO₂@, ZnO core-shell nanoparticles, ACS Applied Materials & Interfaces, 9 (2017) 27784-27791). It is also worthwhile to note that although the introduction of a shell layer potentially enhances the durability of VO₂ nanoparticles and improves the solar luminous transmittance, it may lower the solar modulation ability of the film. For example, the experimental results by Li et al. showed that the solar modulation ability decreased about 50% when the VO₂ nanoparticles were coated by approximately 7 nm thick TiO₂ shells (Y. Li, S. Ji, Y. Gao, H. Luo, M. Kanehira, Core-shell VO₂@TiO₂ nanorods that combine thermochromic and photocatalytic properties for application as energy-saving smart coatings, Scientific Reports, 3 (2013) 1370).

As such, it is desirable to further improve the anti-oxidation ability and increase the lifetime of VO₂ nanoparticles in thermochromic films.

SUMMARY

A process for fabricating a thermochromic film is disclosed which includes the steps of dispersing a predetermined amount of vanadium dioxide (VO₂) nanoparticles in a predetermined amount of poly(methyl methacrylate) (PMMA), stirring a mixture thereof for a first predetermined amount of time, generating crosslinks to molecule chains of the PMMA in the mixture, and blade coating a predetermined thickness of the cross-linked mixture on a substrate to form the thermochromic film. The area and thickness of the fabricated film are 600 mm×300 mm (length×width) and approximately 4 μm, respectively. It is shown that the PMMA-VO₂ film has a luminous transmittance of about 50%, solar modulation ability of about 17.1%, and haze of about 11%. The solar modulation ability of the PMMA-VO₂ film is higher than that of most previous studies which are usually smaller than 10%. The lifetime of the VO₂ nanoparticles is greatly improved by the cross-linked polymer matrix with high molecular weight. The durability tests were performed in the accelerated aging chamber whose environmental temperature is 60° C., and humidity is >95%. It was shown that the lifetime of the VO₂ nanoparticles embedded in the cross-linked PMMA matrix with molecular weight of approximately 950,000 is about 900 hours, which is much longer than that (about 200 hours) of the VO₂ in the non-cross-linked PMMA matrix with low molecular weight (about 15,000). The lifetime of the VO₂ nanoparticles in the highly entangled and cross-linked polymer matrix is close to or longer than the lifetime of thermochromic films made of VO₂ protected by environmental stable materials (e.g., aluminum oxide and SiNx), indicating that cross-linked the polymer chains with high molecular weight could improve the durability of VO₂ remarkably. It was shown that there is no decay of the solar modulation ability after about 3000 cycles of fatigue test. The PMMA-VO₂ film could greatly reduce the cooling demands in hot climates and improve the thermal comfort in cold climates.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A and 1B are flowcharts illustrating a process of producing a durability-enhanced thermochromic film according to an embodiment of the present disclosure.

FIG. 1C is a cross-sectional view of the PMMA-VO₂-on-BoPET film formed by the process depicted in FIGS. 1A and 1B.

FIGS. 2A-2D illustrate characterization of the synthesized VO₂ nanoparticles according to the processes shown in FIGS. 1A and 1B.

FIGS. 3A and 3B illustrate optical performance of the PMMA-VO₂ thin film fabricated according to embodiments of the present disclosure for energy-efficient smart windows.

FIGS. 4A-4D demonstrate the durability performance of the PMMA-VO₂ thin films fabricated according to embodiments of the present disclosure.

FIG. 5A illustrates a setup for fatigue measurements

FIG. 5B illustrates variation of the transmittance of the PMMA-VO₂ film fabricated according to embodiments of the present disclosure as a function of a cycle number.

FIGS. 6A-6C demonstrate thermal comfort and energy saving of the PMMA-VO₂ film of the present disclosure applied to windowpanes in selected cities.

The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein.

DESCRIPTION

The present invention relates to a durability-enhanced thermochromic film with core-shell nanoparticle structures. A preferred embodiment of the present invention will be described hereinafter with reference to the attached drawings.

FIGS. 1A and 1B are flowcharts illustrating a process of producing a durability-enhanced thermochromic film according to an embodiment of the present disclosure. Referring to FIG. 1A, an exemplary process of synthesizing crystalline VO₂ nanoparticles begins with step 102 in which 2.0 grams of V₂O₅ powder is added to 40.0 ml deionized water. In step 105, the V₂O₅/deionized water mixture is stirred for 20 minutes. In step 110, 3.4 grams of oxalic acid dihydrate is added to the mixture. In step 115, the mixture is further stirred until a clear light green or blue slurry is formed. In step 120, the slurry is moved to a 150 ml Teflon-lined stainless-steel autoclave to be kept at 260° C. for 24 hours, and then air-cooled to room temperature. In step 125, the resulting black-precipitates are collected and washed with deionized water and ethanol sequentially for 3 times. In step 130, the washed black-precipitates are dried with a vacuum furnace at 120° C. for 5.0 hours. Then crystalline monoclinic VO₂ nanoparticles are obtained for being embedded in a polymer matrix material. In an embodiment, poly(methyl methacrylate) (PMMA) A4 (molecular weight about 950K from MicroChem Corp) resists in anisole is used.

PMMA is a non-toxic, inexpensive thermoplastic with high optical transparency, high mechanical strength and durability, excellent thermal stability, and weather resistance. PMMA with different molecular weights (chain length and entanglement) could be obtained and processed easily. Moreover, the influence of the near-UV from solar radiation on the PMMA is minor since PMMA is only sensitive to high energy radiation such as the electron beam, x-rays, and UV radiation with wavelengths shorter than 300 nm. Thus, PMMA is exemplarily adopted as a polymer matrix for embedding the VO₂ nanoparticles.

Referring to FIG. 1B, a process of forming a PMMA-VO₂ film begins in step 150, in which a predetermined amount of crystalline monoclinic VO₂ nanoparticles are ultrasonically dispersed into 40 ml of PMMA A4. In step 155, the PMMA-VO₂ solution is stirred for approximately 12 hours. Generally, the molecule chains of PMMA can be interlinked using several methods, including free radical polymerization, condensation reactions, small molecule crosslinking and radiation. In step 160, the PMMA-VO₂ solution is exemplarily irradiated by UV radiation (220˜380 nm, ADJ Products) for about 10 hours in a room environment while being stirred at about 300 rpm to generate crosslinks in the PMMA matrix. The crosslinking degree of the PMMA matrix can be controlled by dose and time of the UV irradiation. Since the PMMA matrix with a higher crosslinking degree usually has a higher endothermic peak, the radiation dose needs to be smaller than 2.0 J/cm² to prevent the brittleness of PMMA film after crosslinking. In step 165, the mixed solution is maintained at a constant temperature of approximately 75° C. on a hotplate for 40 minutes. In step 170, the final solution is then blade coated onto a transparent and chemically stable biaxially-oriented polyethylene terephthalate (BoPET) sheet laminated on a piece of float glass. In an embodiment, the BoPET sheet is approximately 50 μm thick. A final thickness of the dried PMMA-VO₂ film is determined by the concentration of PMMA, the moving speed of the blade and the gap between the blade tip and the BoPET substrate. In an embodiment, the PMMA-VO₂ film is formed using blade coating at a speed of 15 mm/sec. In step 175, the resulting wet film is dried in a fume hood for 2 hours. Then a PMMA-VO₂ film coated on a 50 μm thick BoPET film that can be applied onto windowpane is obtained.

FIG. 1C is a cross-sectional view of the PMMA-VO₂-on-BoPET film formed by the process depicted in FIGS. 1A and 1B. The BoPET film 182 serves as a substrate for the blade coated PMMA-VO₂ film 185 with VO₂ nanoparticles randomly dispersed therein. Although PMMA is exemplarily used as the matrix material for reducing diffusion capacity of gas molecules therein, a skilled in the art may realize that other cross-linked polymer matrix materials can also be used to alleviate the natural oxidation of the embedded VO₂ nanoparticles.

For measuring the PMMA-VO₂ film, the commercial Netzsch differential scanning calorimetry (DSC 204 F1 Phoenix) can be used to determine the phase transition properties of the synthesized VO₂ powder over the temperature range from 0 to 100° C., and the heat absorption properties of the PMMAs from 0 to 550° C., respectively. The heating/cooling rate was set at 10° C./min. The crystalline phase of the VO₂ powder was identified using X-ray diffraction (XRD, MiniFlex600, Rigaku, Japan) with Cu Kα radiation (N=1.5418) at a voltage 40 kV and a current of 40 mA. Transmission electron microscope (TEM, JEM-2010) was used to characterize the morphology and microstructure of the nanoparticles. Optical performance, including spectral transmittance and haze of the films, can be measured using a UV-Vis-Near-IR spectrophotometer (Shimadzu UV-3101) together with a temperature control unit including black anodized aluminum 6061 plate, and a variac (2 A, 120 V). A helium-neon (HeNe) laser (SIEMENS), and a programmable stage temperature controller (LINKAM TMS 94) together with a heating/freezing microscope stage (LINKAM MDS600) with a microscope objective lens (NIKON, Plan Fluor ELWD 20x/0.45) was assembled to measure the reflectance spectrum of the film with a 200 nm silver coating in 633 nm at 25° C. and 90° C. with ramping rate 10° C./min. A TPS (TENNEY) environmental chamber was used to create a steady-state temperature and humidity (60° C., relative humidity around 95%) for the durability testing of the PMMA-VO₂ films.

FIGS. 2A-2D illustrate characterization of the synthesized VO₂ nanoparticles according to the processes shown in FIGS. 1A and 1B. Specifically, FIG. 2A shows the morphology of VO₂ nanoparticles from transmission electron microscopy (TEM). It is seen that most of the nanoparticles have an approximately spherical shape, and the diameter of the VO₂ nanoparticles mainly lies in the range of 50˜80 nm.

FIG. 2B presents an X-Ray Diffraction (XRD) measurement of the VO₂ nanoparticles, where the diffraction peaks indicate that the VO₂ nanoparticles are in the monoclinic phase (M2, C2/m). The phase transition temperature of the VO₂ nanoparticles was determined by differential scanning calorimetry (DSC) measurements using a heating/cooling rate of 10° C./min in the temperature range from 0 to 110° C. Since the insulator-to-metal transformation of VO₂ is a first-order phase transition, the discontinuous variation of entropy results in the release or absorption of latent heat.

As shown in FIG. 2C, when VO₂ transits from the monoclinic (M) insulator phase to the rutile (R) phase during the heating cycle, an endothermic (positive) peak with peak temperature at about 71° C. is observed. A corresponding exothermic (negative) peak, representing metal-to-insulator transition, occurs in the cooling cycle, where the peak temperature is about 63° C. Since the crosslinking process could generate new chemical bonding between neighboring chains, the cross-linked PMMA polymers are expected to have a higher endothermic peak and a larger maximum decomposition temperature.

FIG. 2D compares the DSC thermographs of cross-linked PMMA and non-cross-linked PMMA with molecular weight approximately 950,000. Comparing to the non-UV exposed 950,000 PMMA, the 950,000 PMMA under UV exposure has a higher endothermic peak (from about 6.0 μV/mg to about 9.5 μV/mg) and a larger maximum decomposition temperature (from 375° C. to 386° C.), indicating the crosslinks were generated.

To evaluate the thermochromic performance of the PMMA-VO₂ film, the total transmittance and haze, were both measured using a commercial UV-Vis-Near-IR spectrophotometer (Shimadzu UV-3101). The total transmittance (τ_(λ,tot)) were measured by placing the PMMA-VO₂ film and a diffuse reflector at the inlet and outlet of the integrating sphere, respectively.

FIGS. 3A and 3B illustrate optical performance of the PMMA-VO₂ thin film fabricated according to embodiments of the present disclosure for energy-efficient smart windows. Specifically, FIG. 3A illustrates measured total spectral transmittances of the PMMA-VO₂ film at low temperature (approximately 25° C.) and high temperature (approximately 90° C.). Since the metallic phase VO₂ blocks the near-infrared (NIR) solar radiation, it was found that the transmittance of the PMMA-VO₂ in the range of 0.8˜2.5 μm was much smaller when the temperature of the PMMA-VO₂ film was set at approximately 90° C. The mean luminous (380-780 nm) transmittance τ_(lum) and mean solar (280-2500 nm) transmittance τ_(sol) were then calculated as,

$\begin{matrix} {\tau_{lum} = \frac{\int_{380{nm}}^{780{nm}}{I_{{lum},\lambda}\tau_{\lambda,{tot}}d\lambda\,}}{\int_{380{nm}}^{780{nm}}{I_{{lum},\lambda}d\lambda}}} & (1) \end{matrix}$ $\begin{matrix} {\tau_{sol} = \frac{\int_{280{nm}}^{2500{nm}}{I_{{sol},\lambda}\tau_{\lambda,{tot}}d\lambda\,}}{\int_{280{nm}}^{2500{nm}}{I_{{sol},\lambda}d\lambda\,}}} & (2) \end{matrix}$

where l_(lum,λ) is the standard luminous efficiency function for vision, l_(λ) is the solar radiation intensity of air mass 1.5 (AM1.5) corresponding to the sun standing 37° above the horizon, and τ_(λ) is the transmittance of radiation at wavelength A shown in FIG. 3A. The solar modulation ability Δτ_(sol) is defined by the difference of solar transmittance before and after the phase transition, i.e.,

Δτ_(sol)=τ_(sol)(T<T _(c))−τ_(sol)(T>T _(c))  (3)

According to Eqs. (1-2), the luminous transmittance at low temperature and solar modulation ability are calculated to be approximately 50% and approximately 17.1%, respectively. The sample PMMA-VO₂ film made according to the steps shown in FIGS. 1 , have been checked. The uniformity of the coating by measuring the transmittance of the film at several different locations. The difference of the solar transmittances among different points was found to be smaller than 1%.

Haze is used to characterize the percentage of the transmitted light whose propagation direction deviates a specific angle from the direction of the incident beam. According to the ASTM D1003-13 (ASTM D1003-13-Standard test method for haze and luminous transmittance of transparent, ASTM International West Conshohocken, 2013), haze is defined as,

$\begin{matrix} {H_{\lambda} = {\frac{\tau_{{dif},\lambda}}{\tau_{{tot},\lambda}} = \frac{\tau_{{dif},\lambda}}{\tau_{{dif},\lambda} + \tau_{{dir},\lambda}}}} & (4) \end{matrix}$

where τ_(dif,λ) refers to the light scattered more than 2.5° off from the incident light, and τ_(dif,λ) is the transmitted light within the angle of 2.5°. The diffuse transmittance can be measured by replacing the diffuse reflector at the outlet of the integrating sphere with a light trap to prevent the direct transmittance from influencing the measurement signal. Since the VO₂ nanoparticle size (50 to 80 nm) is much smaller than the wavelength of the visible light (0.4 to 0.8 μm), the scattering of the light passing through the PMMA-VO₂ films can be described by Rayleigh scattering, where the scattering density

${I_{sca} \propto \frac{d^{6}}{\lambda^{4}}},$

where λ is the incident wavelength and d is the diameter of the nanoparticle.

FIG. 3B demonstrates that the haze value decreases as a wavelength of an incident radiation increases. The averaged haze value in the visible range is calculated as,

$\begin{matrix} {H = \frac{\int_{380{nm}}^{780{nm}}{H_{\lambda}I_{{lum},\lambda}d\lambda\,}}{\int_{380{nm}}^{780{nm}}{I_{{lum},\lambda}d\lambda}}} & (5) \end{matrix}$

where H_(λ) is the measured haze value at wavelength λ. According to Eq. (5), the averaged haze in the luminous range is approximately 11%, as shown in FIG. 3B. Note that the haze of the developed film could be lowered by further reducing the size of the VO₂ nanoparticles based on the Rayleigh scattering. In sum, the PMMA-VO₂ thin film fabricated according to the steps shown in FIGS. 1A and 1B has high luminous transmittance (approximately 50%), large solar modulation ability (approximately 17.1%), and relatively low haze visibly (approximately 11%).

Oxygen and moisture in the ambient air could diffuse into the polymer matrix and oxidize the phase-switchable VO₂ to non-switchable V₂O₅, resulting in the loss of solar modulation ability. It is thus desirable to reduce the diffusion capacity of gas molecules in the polymer matrix. Thermal stability, surface hardness, and chemical resistance of the polymer matrix can be improved significantly using entangled and cross-linked molecular chains, which is also directly related to a smaller gas diffusion coefficient.

In practice, the natural oxidation of VO₂ is a long process, and the obvious variation of the thermochromic property in the ambient conditions may only be observed after a few months. Thus, accelerated environmental tests were performed to evaluate the durability of VO₂ nanoparticles. The tests were conducted at a temperature of 60° C., and the relative humidity at approximately 95%. Systematic measurements of spectral transmittance at both low temperature (25° C., insulating phase) and high temperature (90° C., metallic phase) were recorded as a function of time to determine the variation of the thermochromic performance. Each measurement was then repeated at least three times to ensure the testing reliability. To investigate the influence of crosslinking and entanglement on the durability of the PMMA-VO₂ film, the cross-linked and non-crossed-linked PMMA-VO₂ film with two different molecular weights were measured.

FIGS. 4A-4D demonstrate the durability performance of the PMMA-VO₂ thin films fabricated according to embodiments of the present disclosure under the aforementioned accelerated environmental test conditions. Specifically, FIG. 4A demonstrates that the cross-linked PMMA film with molecular weight approximately 950,000 exhibits no noticeable change in optical transmittance after about 200 hours. The thermochromic properties of cross-linked PMMA-VO₂ film with molecular weight approximately 950,000 begin to deteriorate after about 450 hours exposure while still maintaining more than 60% of its solar modulation ability (approximately 10%). After about 900 hours, the solar modulation ability decreased from about 17.1% to about 4.0%, indicating that a large part of the VO₂ nanoparticles was oxidized. For comparison, the durability performances of the non-cross-linked PMMA with molecular weights of approximately 15,000 (FIG. 4B) and approximately 950,000 (FIG. 4C) were also tested under the same accelerated testing conditions (60° C., humidity >95%).

FIG. 4D illustrates the variation of solar modulation abilities of the above three PMMA-VO₂ films as a function of time. It is seen that the decreasing rate of the non-cross-linked film with molecular weight of approximately 15,000 is much faster than that of the non-cross-linked PMMA with molecular weight of approximately 950,000, which is close to decreasing rate of the uncoated VO₂ in the matrix of resin. The thermochromic performance of the VO₂ in the non-cross-linked PMMA with molecular weights of approximately 15,000 disappears after about 200 hours. Furthermore, it is found that the lifetime of the cross-linked 950,000 PMMA was approximately 350 hours longer than that of non-cross-linked 950,000 PMMA at about 600 hours. This indicates that the entangled and crosslinked polymer matrix can substantially improve the lifetime of VO₂ nanoparticles in the PMMA film.

Referring again to FIG. 4D, the lifetime of the PMMA-VO₂ film developed according to the embodiment of the present disclosure is better than the previously reported values for VO₂ nanoparticles coated by SiO₂ (about 72 hours), Al(OH)₃ (about 120 hours) (Y. Chen, X. Zeng, J. Zhu, R. Li, H. Yao, X. Cao, S. Ji, P. Jin, High performance and enhanced durability of thermochromic films using VO₂@ ZnO core-shell nanoparticles, ACS Applied Materials & Interfaces, 9 (2017) 27784-27791), and VO₂ thin film protected by Al₂O₃ (about 100 hours) (Y.-X. Ji, S.-Y. Li, G. A. Niklasson, C. G. Granqvist, Durability of thermochromic VO₂ thin films under heating and humidity: effect of Al oxide top coatings, Thin Solid Films, 562 (2014) 568-573), and is comparable to the performances of VO₂ nanoparticles coated by Al₂O₃ (>480 hours) (K. Tong, R. Li, J. Zhu, H. Yao, H. Zhou, X. Zeng, S. Ji, P. Jin, Preparation of VO₂/Al—O core-shell structure with enhanced weathering resistance for smart window, Ceramics International, 43 (2017) 4055-4061) and VO₂ thin film protected by SiNx (about 600 hours) (S. Long, X. Cao, N. Li, Y. Xin, G. Sun, T. Chang, S. Bao, P. Jin, Application-oriented VO₂ thermochromic coatings with composite structures: optimized optical performance and robust fatigue properties, Solar Energy Materials and Solar Cells, 189 (2019) 138-148). The PMMA-VO₂ film fabricated according to the embodiment of present disclosure has demonstrated an alternative pathway to effectively improve the lifetime of the VO₂ nanoparticles, avoiding the emergence of cracks induced by the periodic insulator-to-metal phase change found in the core-shell structures.

Fatigue tests are used to determine the numbers of cycles (fatigue life) that a material or structure can withstand under cyclic loadings. The emergence of cracks in VO₂-based films or complete fractures may occur due to the lattice transformation of VO₂ during many cycles of phase transitions from the insulating to the metallic state. Therefore, the fatigue test is performed to study the stability of the PMMA-VO₂ thermochromic film developed according to embodiments of the present disclosure.

FIG. 5A illustrates a setup of fatigue test for the PMMA-VO₂ film fabricated according to embodiments of the present disclosure. A PMMA-VO₂ sample 502 is coated on an approximately 100 nm thick bottom silver layer 505. The resulting film is then placed on a programmable stage 508 which is temperature controlled a computerized controller 520 such as a LINKAM MDS600. The programmable stage 508's temperature ramping rate is set at 15° C./min and an extra 45 seconds delay was assigned to stabilize the temperature of the film at both 25° C. and 90° C. Meanwhile, the film was constantly exposed to a 633 nm focused laser beam from a helium-neon (HeNe) laser 513. The intensity of the transmitted light (μW) was then recorded by one or more detectors 517 and a computer 530 at both low temperature (25° C., insulating phase) and high temperature (90° C., metallic phase) to complete a cycle.

FIG. 5B illustrates a relationship between the number of cycles and the intensity of the transmitted light. As shown in FIG. 5B, there is no noticeable change in transmitted laser intensity in both the metallic and insulator phase after 3000 continuous cycles, indicating that the solar modulation ability of the cross-linked PMMA-VO₂ remained constant. To ensure the reliability of the test, the measurements were repeated at another two sample locations during each cycle and the results in FIG. 5B are an arithmetic average of the three tested points. Note that since incident light traveled across the film twice, the difference between the metallic phase (25° C.) and the insulator phase (90° C.) in FIG. 5B is approximately twice larger than that of FIG. 3A.

FIGS. 6A-6C illustrate thermal comfort and energy saving of the PMMA-VO₂ film of the present disclosure applied to windowpanes in selected cities. Specifically, FIG. 6A depicts a heat and solar radiation transfer pattern through a windowpane. A heat transfer model is developed to evaluate the energy saving performance of a PMMA-VO₂ film in different climates and regions. Here, a PMMA-VO₂ film with a thickness of about 4 μm is employed. The spectral transmittance of the PMMA-VO₂ film in both the insulating phase and the metallic phase was shown in FIG. 3A. Since the thickness of the window is much smaller than its width and length, the heat transfer in the z-direction can be treated as one-dimensional. Thus, the heat transfer in the z-direction can be described as,

$\begin{matrix} {{{k_{gla}\frac{\partial^{2}{T(z)}}{\partial z^{2}}} + {\nabla{q(z)}}} = 0} & (6) \end{matrix}$

where k_(gla) is the thermal conductivity of the float glass, T(z) is the temperature at position z, q(z) is the local heat source carried by the solar irradiation, which can be written as

q(z)=τ_(sol) I _(o) e ^(−β) ^(gla) ^(z)  (7)

where I_(o) is the incident solar energy, τ_(sol) is the solar transmittance of the PMMA-VO₂ film, and β_(gla) is the extinction coefficient of the float glass. Compared with the thickness of float glass (3 mm), the thickness of the PMMA-VO₂ (about 4 μm) film can be ignored. Thus, the boundary conditions at z=0 and z=L_(gla) are as follows:

.q(z=0)=h _(o)(T _(a) −T _(z=0))+σe _(o)(T _(a) ⁴ −T _(z=0) ⁴)+Q _(abs)  (8)

q(z=L _(gla))=h _(i)(T _(z=L) _(gla) −T _(r))+σε_(i)(T _(z=L) _(gla) ⁴ −T _(r) ⁴)  (9)

where T_(a) and T_(r) are the external ambient temperature and the internal room temperature, respectively, h_(e), h_(i) and ε₁, ε_(i) are the external and internal convective heat transfer coefficients and average external and internal surface emissivities, σ=5.67×10⁻⁸ Wm/K⁴ is the Stefan-Boltzmann's constant, Q_(abs)=σ_(sol)I_(o), is the absorbed solar irradiation by the PMMA-VO₂ film, and α_(sol) is the absorbance of the PMMA-VO₂ film. Here, Eq. (6) was solved by the finite volume method. The thermal conductivity and average surface emissivity of the float glass are assumed to be k_(q)=0.96 W/(mK) and ε_(o)≈0.84 according to reference (B. P. Jelle, Solar radiation glazing factors for window panes, glass structures and electrochromic windows in buildings-Measurement and calculation, Solar Energy Materials and Solar Cells, 116 (2013) 291-323). The surface emissivity of the PMMA-VO₂ film is ε_(o)≈0.9. From FIG. 3A, the solar transmittances (τ_(sol)) of the PMMA-VO₂ film in the insulating phase and metallic phase are 0.57 are 0.40, respectively. The reflectances (r_(sol)) of the PMMA-VO₂ in both the insulating and metallic phases were measured as 0.05. Thus, the absorbance can be calculated by α_(sol)=1−τ_(sol)−r_(sol). The inside and external convective heat transfer coefficients can be evaluated by h_(i)=3.6 W/(m²K) and h_(o)=(10+4.1ν) W/(m²K), where ν (m/s) is the wind speed. The weather conditions including ambient temperature (T_(o)), window speed (ν) and solar irradiation density (I₀) are acquired from the NSRDB Data Viewer (https://maps.nrel.gov/nsrdb-viewer/).

FIG. 6B is a plot comparing the inside surface temperatures of a single-pane window with and without a PMMA-VO₂ film. After employing the PMMA-VO₂ film, the inside surface temperature of the single-pane window is approximately 5° C. higher than that of the single-pane window without such a film, indicating that the PMMA-VO₂ film can reduce the temperature difference between the occupants and the innermost surface of windows, which indeed improves the thermal comfort and condensation resistance of the single-pane window in cold climates significantly. Since the solar transmittance of the PMMA-VO₂ film (0.57˜0.4 according to FIG. 3A) is smaller than the single-pane window (approximately 90%), applying the PMMA-VO₂ film increase the heating loads in cold climates and reduce the cooling loads in hot climates.

FIG. 6C is a chart comparing reduced annual heating/cooling loads of a south-facing window in different regions. In FIG. 6C, the positive/negative sign represents the reduced/increased energy. For simplicity, it is assumed that the transmitted solar energy decreases (increases) the heating (cooling) loads when the external ambient temperature is higher (lower) than the internal room temperature (21° C.). Note that the transmitted solar energy includes both the directly transmitted solar irradiation and the absorbed solar energy by window pane and subsequently transferred towards the indoor room environment through heat conduction. Clearly, the cooling demands in the hot areas (e.g., Phoenix, AZ) are significantly reduced. However, in the cold areas (e.g., Boston, MA), the increased heating loads in winter are much higher than the reduced cooling loads in summer, indicating that the PMMA-VO₂ film could increase the annual energy cost.

Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims. 

What is claimed is:
 1. A thermochromic film comprising: a substantially transparent substrate; and a polymer matrix layer coated on the substrate, the polymer matrix layer having cross-linked molecule chains generated before the coating, and the polymer matrix layer being embedded with randomly dispersed thermochromic nanoparticles before the coating.
 2. The thermochromic film of claim 1, wherein the substrate is a biaxially-oriented polyethylene terephthalate (BoPET) sheet.
 3. Thermochromic film of claim 1, wherein the thermochromic nanoparticles are crystalline vanadium dioxide (VO₂) nanoparticles.
 4. The thermochromic film of claim 3, wherein the crystalline vanadium dioxide (VO₂) nanoparticles are synthesized from vanadium pentoxide (V₂O₅) powder.
 5. The thermochromic film of claim 4, wherein the synthesizing process comprises: adding a predetermined amount of vanadium pentoxide (V₂O₅) powder to a predetermined amount of deionized water to form a first mixture; stirring the first mixture for a first predetermined amount of time; adding a predetermined amount of oxalic acid dihydrate to the first mixture to form a second mixture; further stirring the second mixture until a clear light green or blue slurry is formed; keeping the second mixture at a first predetermined elevated temperature for a second predetermined amount of time; air-cooling the second mixture to room temperature to obtain black-precipitates; washing the black-precipitates with deionized water and ethanol sequentially; and drying the washed black-precipitates with a vacuum furnace at a second elevated temperature for a third predetermined amount of time.
 6. The thermochromic film of claim 5, wherein the first predetermined amount of time is approximately 20 minutes.
 7. The thermochromic film of claim 5, wherein the first elevated temperature is approximately 260° C.
 8. The thermochromic film of claim 5, wherein the second amount of time is approximately 24 hours.
 9. The thermochromic film of claim 5, wherein the second elevated temperature is approximately 120° C.
 10. The thermochromic film of claim 5, wherein the third predetermined amount of time is approximately 5 hours.
 11. The thermochromic film of claim 3, wherein the polymer matrix layer is produced by dispersing a predetermined amount of the thermochromic nanoparticles in a predetermined amount of a polymer matrix material to form a third mixture, and stirring the third mixture for a fourth predetermined amount of time.
 12. The thermochromic film of claim 11, wherein the fourth predetermined amount of time is approximately 12 hours.
 13. The thermochromic film of claim 11, wherein ultrasonic energy is applied to the polymer matrix material during the dispersing.
 14. The thermochromic film of claim 11, wherein the polymer matrix material is poly(methyl methacrylate) (PMMA) with a molecular weight of approximately 950,000 forming a matrix.
 15. The thermochromic film of claim 14, wherein a lifetime of the crystalline vanadium dioxide (VO₂) nanoparticles embedded in the cross-linked polymer matrix material is longer than approximately 900 hours under a temperature of approximately 60° C., and a relative humidity of approximately 95%.
 16. The thermochromic film of claim 11, wherein the cross-linked molecule chains are generated by irradiating the third mixture with a radiation for a fifth predetermined amount of time while stirring the third mixture at a predetermined rev per minute (rpm).
 17. The thermochromic film of claim 16, wherein the radiation is a UV radiation with a wavelength shorter than 300 nm.
 18. The thermochromic film of claim 16, wherein a dose of the radiation is adjusted to control a cross-link degree in the polymer matrix material.
 19. The thermochromic film of claim 16, wherein the fifth predetermined amount of time is approximately 10 hours.
 20. The thermochromic film of claim 16, wherein the predetermined rev per minute (rpm) is approximately
 300. 21. The thermochromic film of claim 16, wherein the irradiated third mixture is maintained at a substantially constant temperature of approximately 75° C. for approximately 40 minutes.
 22. The thermochromic film of claim 1, wherein the polymer matrix layer is coated on the substrate using a blade moving relative to the substrate at a predetermined speed while maintaining a predetermined gap to the substrate.
 23. The thermochromic film of claim 1, the polymer matrix layer coated substrate is dried in a fume hood for approximately 2 hours. 